In a photometer used to measure, for example, soot levels in lubrication oils, the amount of oil in waste water, and the like, a source directs infrared radiation through a sample and to one or more detectors. See, for example, U.S. Pat. Nos. 2,648,249; 3,937,962; 3,818,198; 5,041,723; 5,734,165; and 6,420,708 incorporated herein by this reference and P. A. Wilks “In-line Infrared Sensors Covering the Mid Infrared from 2 to 14 μm (5000 to 700 cm−1)”, Vibrational Spectroscopy-Based Sensor Systems, Steven D. Christesen, Arthur J. Sedlacek III, Editors, Proceeding of SPIE Vol. 4577 (2002), also incorporated herein by this reference.
Pyroelectric detector elements may be placed on top of the base of a TO can package and each detector element is associated with a different filter. See also U.S. Pat. No. 4,943,800 incorporated herein by the reference. In operation, a pyroelectric detector responds to a temperature change caused by incident infrared radiation from the source passing through the sample by generating a small electric charge. If, for example, oil in a water sample absorbs radiation at the wavelength λoil and a pyroelectric detector is filtered such that only wavelength λoil reaches the detector, the λoil wavelength radiation will not reach the pyroelector detector and it will not generate an electric charge (or it will generate a lesser charge). But, since the pyroelectric detector responds to any changes in temperature, a change in the ambient temperature will cause the output of the pyroelectric detector to change resulting in an erroneous reading.
To account for fairly large ambient temperature changes, a temperature probe (e.g., a thermistor) can be attached to the TO can and changes in ambient temperature can be compensated for during calibration of the photometer.